U.S Pat. Nos. 4,859,858 and 4,859,859, both entitled GAS ANALYZERS, were issued to Knodle et al. on 22 Aug. 1989. These patents disclose state-of-the-art apparatus for outputting a signal indicative of the concentration of a designated gas in a sample being monitored by the apparatus. These patents are hereby incorporated by reference thereto into this application.
The gas analyzers disclosed in the '858 and '859 patents are of the non-dispersive type. They operate on the premise that the concentration of a designated gas can be measured by: (1) passing a beam of infrared radiation through the gas, and (2) then ascertaining the level of the attenuated energy in a narrow band absorbable by the designated gas. This done with a detector capable of generating a concentration proportional electrical output signal.
One important application of the invention at the present time is in capnometers for monitoring the level of carbon dioxide in the breath of a medical patient. This is typically done during a surgical procedure as an indication to the anesthesiologist of the patient's condition. As the patient's well being, and even his life, is at stake, it is of paramount importance that the carbon dioxide concentration be measured with great accuracy.
In a typical instrument or system employing non-dispersive infrared radiation to measure gas concentration, including those disclosed in the '858 and '859 patents, the infrared radiation is emitted from a source and focused by a mirror on the gases being analyzed. After passing through the body of gases, the beam of infrared radiation passes through a filter. That filter absorbs all of the radiation except for that in a narrow band centered on a frequency which is absorbed by the gas of concern. This narrow band of radiation is transmitted to a detector which is capable of producing an electrical output signal proportional in magnitude to the magnitude of the infrared radiation impinging upon it. Thus, the radiation in the band passed by the filter is attenuated to an extent which is proportional to the concentration of the designated gas. The strength of the signal generated by the detector is consequently inversely proportional to the concentration of the designated gas and can be inverted to provide a signal indicative of that concentration.
The NDIR gas analyzers disclosed in the '858 and '859 patents employ an infrared radiation emitter which has a layer of an electrically resistive, emissive material on a substrate fabricated from a material with low thermal conductivity such as steatite.
For a gas analyzer of the NDIR type to operate efficiently, the infrared radiation passed through the gas undergoing analysis must be of a modulated or pulsed character. That is, it must be made available as a stream of pulses rather than in the form of a continuous beam. Otherwise, the signal-to-noise ratio of the electrical signal available from the analyzer will typically not be high enough for the signal to be useful.
Two methods for supplying modulated infrared radiation to the environment in which sampling occurs have heretofore been proposed. One employs a mechanical chopper to create the wanted stream of pulses from a continuous beam of energy outputted by the infrared radiation source. A chopper has a spinning wheel between the infrared radiation source and the detector. The wheel has a series of apertures spaced equally around its periphery. Consequently, as the wheel rotates, the transmission of the attenuated beam of infrared radiation to the detector of the gas analyzer is alternately enabled and interrupted, typically at a frequency of less than one hundred cycles per second.
Gas analyzers of the character just described are disclosed in U.S. Pat. No. 3,793,525 issued Feb. 19, 1974, to Burch et al. fir DUAL-CELL NON-DISPERSIVE GAS ANALYZER; U.S. Pat. No. 4,811,776 issued May 21, 1974, to Blau, Jr. for GAS ANALYZER; U.S. Pat. No. 3,987,303 issued Oct. 19, 1976, to Stoft et al. for MEDICAL ANALYTICAL GAS DETECTOR; U.S. Pat. No. 4,011,859 issued Mar. 15, 1977, to Frankenberger for METHOD FOR CONTINUOUSLY MEASURING THE CO.sub.2 CONTENT IN BREATHING GAS; U.S. Pat. No. 4,204,768 issued May 27, 1980, to N'Guyen for GAS ANALYZERS OF THE SELECTIVE RADIATION ADSORPTION TYPE WITH A CALIBRATION CELL; U.S. Pat. No. 4,268,751 issued May 19, 1981, to Fritzlen et al. for INFRARED BREATH ANALYZER; AND U.S. Pat. No. 4,371,785 issued Feb. 1, 1983, to Pedersen for METHOD AND APPARATUS FOR DETECTION OF FLUIDS and in a Reliable, Accurate CO.sub.2 Analyzer for Medical Use, Solomon, HEWLETT-PACKARD JOURNAL, Sep. 1981, pages 3-21.
Gas analyzers with mechanical choppers have a number of drawbacks. They are bulky, heavy, and expensive; have moving parts, which is undesirable; and also have complex optical designs. They also tend to be less accurate than is desirable and to lack long term stability.
Also, gas analyzers employing mechanical choppers are relatively fragile. For example, they will typically not work properly, if at all, after they are dropped.
A second, and we believe superior, method of generating the necessary modulated infrared radiation is to drive the infrared radiation emitter with a power supply which applies pulses of electrical energy to the emitter. In an NDIR analyzer with the type of infrared radiation emitter described above, an emissive layer of the emitter heats up and emits a pulse of infrared radiation when a pulse of electrical energy is applied to that layer by the power source. Thereafter, the emissive layer rapidly cools down. Therefore, in the remainder of the emitter duty cycle, radiation in the infrared portion of the spectrum is not outputted from the emitter to any appreciable extent.
This modulation technique has the advantage of eliminating mechanical systems and components along with the attendant bulk, weight, complexity, and fragility of those devices. Also, the pulsed power supply approach tends to be significantly more accurate than the mechanical chopper technique.
Heretofore, the power supplies used for the purposes just discussed and in similar applications supplied unipolar pulses of controlled magnitude, duration, and frequency to the driven emitter--that is, a stream of pulses all having either a positive (or negative) value. As a result, pulsed emitters tend to have an important disadvantage common to schemes employing mechanical choppers, a lack of long-term stability. This is a result of the operating current always flowing in the same direction through the emitter and producing strong electrical fields. These, because they are all oriented in the same direction, cause migration of the emitter materials over time. The consequence of that migration is degraded performance and, ultimately, failure of the emitter.